The gate current of Al27Ga73N/GaN heterogeneous field effect transistors (HFETs) is investigated using current-voltage (IV) and current-temperature (IT) measurement demonstrating that trap assisted tunneling (TAT) is the primary current mechanism. Excellent fit to experimental data is achieved using a thermionic trap assisted tunneling (TTT) model. A single value for each of the primary parameters (Schottky barrier height, trap energy, donor density and trap density) results in a sigma of 1.38x10-8 A for IT data measured at five voltages between 85K and 290K and for IV data measured at three temperatures between 0.0 V and -4.0 V. High energy (gt;0.5 MeV) neutron irradiation at fluences between 4.0x1010 and 1.2x1012 n/cm2 confirms an increase of gate current with fluence. A change in IV characteristics, interpreted as an increase in magnitude of threshold voltage, is also observed.